Radiator my Kawasaki GPZ 400 R overheat after only short term little hard riding, and them let out water from reserviortank. is this normal?

Hi and welcome to FixYa,

Depending on the condition/age of the engine, riding habits, weather condition and ambient temperature, it should not. excessive water/coolant draining out is an overheat condition while a trickle or a few drops would be slightly beyond normal.

Still, best would be to check:

• possibility of clogged radiator;
• ratio of water to coolant;
  
• blower/fan not working;
• thermostat stuck in closed position or restricted;
• broken vanes from the water pump.

Hi m8,, hopefully u dont have a white frothy lookin gloop in the radiator ,if u have then the head gasket has gone,, if u havent then check the thermostat is working properly,, u can check this by taking the thermostat out ,and boiling sum water in a pan ,,dip the thermostate in the water to see if it opens out,,if it doesnt u need a new one,, or as a quick test,, take the thermostat out ,and put the housing back to gether with out the thermostat in,, and start her up, see if the bike runs cooler ,, try this 1st m8,, if no change come back and il take u threw the next step,,cheers